Conventionally, a boiler used in vapor production or power generation is equipped with numerous water pipes therein, supplies water thereto, obtains vapor at high temperature and pressure, and uses the same as a heat source or a steam turbine.
In particular, in case of power generation, as efficiency of Rankin cycle using the steam turbine increases as temperature and pressure of steam increase, it is critical to obtain steam at high temperature and pressure to improve efficiency of power generation.
As an example, in case of a pulverized coal boiler for coal thermal power generation that is currently commercialized adopts the ultra super critical (USC) conditions of main steam pressure of 241 bar, main steam temperature of 566° C., and reheat steam temperature of 593° C. For an improved efficiency, a boiler suitable to hyper-super critical (HSC) condition of steam temperature of 700° C. or higher is under development.
Likewise, in order to generate steam at a high temperature and pressure, numerous heat medium injection parts (or heat exchange tubes) need to be installed inside the boiler for steam production. In particular, properties of the material of the heat medium injection part of a high temperature part which can endure a steam condition of increasing temperature and pressure as they increase are important. In case of a USC boiler, very high-cost steel is used, but a material capable of enduring much higher temperature and pressure of steam for a HSC boiler has not been commercialized.
At a part where the steam is maintained at high temperature and pressure, heat transfer becomes particularly important and durability of welding and corrosion resistance of the heat medium injection part are considered very important during production process, causing a high price of the boiler. In most cases, however, the heat medium injection part breaks down, leading to a huge damage.
Meanwhile, there has recently been an effort in using carbon dioxide instead of water as a working medium of a boiler all over the world including South Korea. Carbon dioxide has its critical temperature and pressure at 31.04° C. and 72.8 atm, respectively, and becomes supercritical state at a temperature below supercritical temperature and pressure of water at 374° C. and 218 atm, respectively. Additionally, it is known that the turbine using the supercritical CO2 as a working medium has an effect of decrease in size compared to the steam turbine.
However, a material capable of enduring supercritical CO2 at high temperature and pressure when using CO2 in a conventional boiler is still under investigation.
As an example of conventional technology, Korean Patent Publication No. 2010-0104563 discloses a boiler which indirectly heats the heat medium injection part using a working medium such as oil to solve a problem caused by directly heating the heat medium injection part. However, as a boiling point of the oil as a medium for heating the heat medium injection part is between about 250° C. and 350° C., it is impossible to make the heat medium be in a supercritical or ultra supercritical state as described above.